Portable, robust optical

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Portable, robust optical Frequency standards in
hollow optical fiber
Mohammad Faheem, Rajesh Thapa, Ahmer Naweed,
Greg Johnson, and Kristan Corwin
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• Motivation
• Develop high accuracy Portable Wavelength
  Standards for Telecommunication Industry.

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Outline
Introduction
Broadening Mechanisms and Saturation Spectroscopy
Frequency measurement.
Previous Work
Our Approach
Experimental Set-up
Results
Limitations
Future Work
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Over View
Laser
50 Torr
?
Frequency measurement Frequency comb
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Wavelength Division Multiplex
Separation between channel is 1 nm
- Channel adjustment in WDM (1525-1565 nm)
system. - Calibration of
wavelength measurement devices.
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Recipe for a Wavelength Standard

• Atomic or molecular absorption lines
• - Absolute frequency reference.
• - Very stable under changing environmental
• conditions
• - Good references in the 1500 nm
  region
• Good references in 1500 nm region
• Acetylene (1510-1540 nm)
• Hydrogen cyanide (1530-1565 nm)
• Rubidium (1560-1590 nm)

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Acetylene ( Also called Ethyne)
H
C
H
C

• - Colorless and extremely flammable
• 50 transition lines, spaced by 60-80 GHz
  extending from
• 1515nm-1540 nm

C
C
H
H
CH
n1
Symmetric C- H bond Stretching
CC
n2
n1n3 lies in 1.5 mm region
CH
n3
Anti-symmetric C- H bond Stretching
Stretching Vibrations
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Important Broadening In IR Spectroscopy

• Absorption

(Beers law)
z
a is the absorption cofficient
Absorption sample

• Power Broadening

• Doppler Broadening

C2H2 at room temp. 500 MHz
Laser spectroscopy by Wolfgang Demtroder
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Important broadening near IR spectroscopy

• Pressure broadening
• and shift

Line Broadening
C2H2 broadening P(10) 11.6 MHz/Torr
Line Shift
C2H2 line shift P(10) 0.29 MHz/Torr

• Transit-Time Broadening

Transit time Td/v
500 KHz for 0.94 mm dia cavity
Laser spectroscopy by Wolfgang Demtroder
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Saturation Spectroscopy

• Eliminates Doppler width
• Requires high Power (Typically 300 mW
• for acetylene)
• Dominant Line width
• Pressure broadening (11 MHz/Torr)
• Transit-time broadening (coherence
• time between laser and molecules)
• Power Broadening
• Signal Size
• Depends linearly on pressure
• Depends linearly on sample length

Laser
10
90
B.S
Pump Beam
Probe Beam
M2
M1
Cell
Det.
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Frequency Measurement

• Frequency Cycles/second
• Definition of time

Caesium 133 atom
Duration of 9 192 631 770 period Of the
radiation corresponding to the transition
between two hyperfine Level of the ground state
of Cs atom.
Optical frequency ----- In hundreds of THz
Its easy to measure in THz ?
Photo Detector ------ In 40-100 GHz
What we need to do?
Mode Locking
Frequency Comb
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Time-Frequency Correspondence
Fourier transform of periodic signal
discrete frequency components.
I(f)
fo
fr
f
fn n fr fo
0
fr Laser repetition rate fo
Offset
D. J. Jones, et al. Science 288, 635 (2000)
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Measurement of fr and fo
Repetition Rate
fr can be measured with photo-detector in
optical path
Offset
I(f)
fo
fr
f
f2n
0
fn
2fn-f2n 2(nfrepfo) - (2nfrepfo) f0
Octave Spanning
- Microstructure fiber - Laser Cavity
D. J. Jones, et al. Science 288, 635 (2000)
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A.Czajkowski,J.E Bernard,A.A.Madej,R.S.Winler
Self reference frequency comb
Unknown signal
fr
fo
f
Unknown signal
App.Phys.B79,45-20 (2004)
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Solid core microstructure Fiber
Fused silica core
Cladding
Core
1.7 ?m
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Frequency comb Set-up
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Previous work K.Nakagawa, M.de Labachelerie,
Y.Awaji and Kourogi
(J.opt.soc.Am.B/Vol.13,No.12/December1
996)
Cavity

• Long interaction length.
• High intracavity power (100 mw).
• Fragile.
• Cavity and laser locked to
• resonance independently.

Signal Measurement

• Two photon Rb (778 nm) transition as
• a reference.
• Hydrogen Cyanide(1556 nm, P(27)) as a
• Intermediate reference.

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Previous Work W.C. Swann and S.L. Gilbert.
(NIST) Pressure-induced shift and broadening of
15101540-nm acetylene wavelength calibration
lines, Opt. Soc. Am. B,
17, 1263 (2000).
Pressure broadening shift For P(13)
broadening 11.4 MHz/Torr Line
shift 0.27 MHz/Torr Effect of Temp
negligible effect Used to calibrate Optical
Spectrum Analyzers (OSAs)
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Previous Work A.Czajkowski, A.A.Madej,
P.Dube Development and Study of a 1.5 um
Optical frequency Standard referenced to p(16)
Saturated absorption line in the (V1v3)
overtone band of 13C2H2
Optics Communications 234(2004) 259-268
Saturation signal 1 MHz Measure Power shift
11.4 Hz/mw Pressure Shift 230 Hz/mTorr
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Our Approach
Develop high accuracy portable wavelength
Standards for telecommunication industry.
Through existing Technology
- Cavity based references are not Portable. -
Transitions in the glass cells can not be further
narrowed.
Solution
Use molecular absorption inside optical fiber.
Advantages

• Portable
• Easy to align
• Easier to get high intensities over long path.

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Experimental Set-up
ultimately
Fiber in
Fiber out
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Setup- Optics
PD
Diode Laser
Fringe width156 MHz
Mirror
50/50
d2
d1
Mirror
BS
PBS
Pump Beam
Probe Beam
10/90
Fiber
EDFA
30/70
PBS
PD
ISO
Probe
Squeezer
PBS
PD
ISO
Pump
Squeezer
?/2
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Capillary Tube
1531.31 nm

• Too lossy

Length 18 cm and dia 330 µm

• Only 40 transmission
• Doppler Broadened signal
• observed
• No saturation signal.

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Capillary tube
Satisfy Beers law
10 µm PBF gives 50 MHz saturation dip. 300 µm
should give 1.73 MHz saturation dip.
For saturation dip We need power 865 times
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Photonic Bandgap fiber
10 mm
losslt 0.02 dB/m
No total internal reflection Braggs reflection
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10-µm Photonic Bandgap fiber
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10-µm Photonic Bandgap fiber
1.2 Torr of 12C2H2 at 1531.31 nm
Significant signal strength at 10 and 20 mW pump
powers!
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10-µm Photonic Bandgap fiber
We are transit limited or pressure limited ?
Line width does not increase significantly with
pressure which implies that it is transit time
limit.
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20-µm Photonic Bandgap fiber
20 mm core, 60 cm length Fiber fills to 2 mTorr
in 10 s
20 µm, 83 cm long PBF at 1531.20 nm
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20-µm Photonic Bandgap fiber
Pressure limited ?
Factor of 3 change in pressure gives a factor of
1.2 change in line width
Transit limited
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10-µm, 20- µm PBF data Comparison
Transit Time
To reduce Transit time Broadening increase
fiber hole size -or- find a heavier molecule -or-
Decrease the velocity of molecule by
cooling
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Ultimate limits

• Signal strength
• optimal fiber length for pressure.
• Noise
• Interference (probe with stray/reflected pump)
• laser intensity noise
• Linewidth (target lt 1 MHz)
• transit time broadening
• pressure-broadening
• To narrow the transition, we must
• reduce transit-time broadening
• reduce the pressure
• lengthen the fiber

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Conclusions
- Observed saturated absorption features in
photonic bandgap fiber for first
time. - Significant absorption fraction
observed at low power (lt 20 mW), with 23
MHz-wide feature. - Confirmed transit time
broadened, 20 mm produce narrower
feature than 10 mm fibers
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Future Plan

• Near-term
• Make more portable, reduce noise.
• Build frequency Comb for absolute measurement.
• Observe dependence of different broadening
• mechanisms.
• Observe the shifts in Photonic bandgap fibers.
• Longer-term
• Seal the fiber filled with gas. (Greg Johnson)
• Narrow the transition
• Explore larger photonic bandgap fibers
• Explore other gases.

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Thank You
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Photonic Bandgap fibers
Index guiding
Hollow Core guiding
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Saturated absorption feature width
40 MHz
Transit time broadening Naive estimate t d/v
1/50 MHz Pressure broadening 11 MHz/Torr 1.2
Torr 13.2 MHz
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Important Broadening In IR Spectroscopy

• Doppler Broadening
• Molecules are in motion when they absorb energy.
  This causes a change in
• the frequency of the incoming radiation.
• Pressure broadening
• Produce by the shifts of energy levels by
• interaction of radiating atom with near by
• particles
• Transit time Broadening
• The interaction time of molecules with the
• radiation field is small with the
• spontaneous life time of excited levels
• Power Broadening
• Molecules absorb energy from intense laser. This
  causes a energy shift causing broadening.